Resists of the chemical amplification type are characterized by their patterning reaction in which the decomposition of acid-decomposable functional groups or the crosslinking of acid-crosslinkable functional groups is promoted by using as a catalyst a strong acid produced by exposure in a thermal treatment carried out after the exposure (the so-called post exposure bake, which is abbreviated as PEB). Such resists can be applied to various types of lithographic arts, e.g., excimer lithography, electron-beam lithography, X-ray lithography and so on. As they are sensitized by the use of the catalytic reaction, the resists of the foregoing type are particularly suitable for electron-beam lithography and X-ray lithography in which high sensitivity is required.
In the lithographic arts, the standard of requirement for sensitivity is 1 .mu.C/cm.sup.2 with respect to electron-beam resists, while it is 100 mJ/cm.sup.2 with respect to X-ray resists. It is a well-known fact that a resist responsive to electron beams has responsiveness to X rays also. Actually, there is a good correlation between the electron-beam sensitivity and the X-ray sensitivity. Although these sensitivity values depend on the characteristics and the condition of light sources used, it can be roughly estimated that the electron-beam sensitivity of 1 .mu.C/cm.sup.2 corresponds to the X-ray sensitivity of 50-100 mJ/cm.sup.2.
As far as attention is paid to the sensitivity alone, it is easy for the resists of chemical amplification type to fulfill the foregoing standard requirement since the sensitivity can be freely controlled by changing the PEB condition. However, resists cannot serve a practical purpose unless requirements for all the characteristics thereof, namely not only sensitivity but also other various characteristics including resolution, dimensional control, heat resistance, dry etching resistance, storage stability and film-thickness control, are cleared up. Further, the application target in electron-beam lithography and X-ray lithography has much finer dimensions than the lower limit thereon in light lithography. Accordingly, the requirement for each characteristic becomes extremely severe. Such being the case, it is the present situation that previously proposed positive resists for electron-beam or X-ray lithography have not succeeded in bearing characteristics excellent enough for practical use.
Moreover, positive resists of the chemical amplification type have the problem that their acid-decomposable protective groups are gasified through the decomposition during the base-plate processing with plasma or the UV curing as the pretreatment of the base-plate processing, and so they tend to cause a decrease in film thickness and form a distorted pattern. For instance, a two-component positive resist of the type which contains poly(p-t-butoxycarbonyloxystyrene) (PBOCST) as a main component in the proportion of, e.g., 90 wt % suffers a weight loss of, e.g., greater than 40 wt % because t-butoxycarbonyl (t-Boc) groups comprise 45% of the PBOCST's weight. As a change in resist density is just a little, on the other hand, the loss in weight gives rise to a decrease in film thickness and the formation of a distorted pattern. This phenomenon is a serious drawback. Not only PBOCST has this drawback, but also other two-component positive resists have it in common, because the amount of protective groups introduced therein is large in general.
Also, three-component resists encounter the above-described problem, For instance, the three-component resists which we developed before had a weight loss of about 9% through the decomposition of the protective groups during the UV curing and a marked distortion with respect to the hole pattern wherein each hole is encircled with the resist. Occasionally, we observed in the foregoing resists a distortion of 0.1 .mu.m or greater. Thus, it has turned out that, even if the proportion of gasifiable protective groups to the resist as a whole is reduced to 9 wt %, the resist is not adequate for fine processing on the level of 0.2 .mu.m.
A definite guideline on the limit of an allowable weight loss in the resist due to decomposition and gasification of the protective groups contained therein has not yet been laid down by anyone skilled in the arts, In electron-beam and X-ray lithographic processes however, it is required that the dimensional accuracy be on a level higher than in conventional photolithography. Therefore, the weight loss in a conventional photoresist of diazonaphthoquinone-novolak type can be adopted as an index to the positive resists for electron-beam or X-ray lithography. More specifically, since the weight loss in the photoresist due to the elimination of nitrogen through the photochemical reaction is at most 3-4%, although it depends on the structure of the diazonaphthoquinone compound used and the content thereof, the weight loss due to the removal of protective groups should be controlled to less than 4% in the positive resists of chemical amplification type, too.
As the weight loss is determined primarily by the amount of protective groups introduced, the foregoing limitation on weight loss can easily be attained so far as the sacrifice of resolution and residual film characteristics is made, and so some known positive resists of chemical amplification type have already succeeded in weight loss control from the theoretical point of view. However, the fact is that such limitation is not yet fulfilled in any compositions which have various resist characteristics excellent enough to serve a practical purpose.
In addition, ensuring heat resistance is an important problem. In the case of a photoresist of the diazonaphthoquinone-novolak resin type, the heat resistance is substantially raised by the development with an alkaline aqueous solution, because the diazonaphthoquinone compound and the novolak resin are linked together by the diazo coupling reaction which takes place during the development. However, conventional positive resists of the chemical amplification type were devoid of such a mechanism. When a novolak resin was used therein as a main component, therefore, the resulting resists tended to cause a thermal flow during the base-plate processing with plasma. Thus, a problem of thermal resistance was posed to the positive resists of the type which contained a novolak resin as a main component. Although an attempt to improve the heat resistance of such resists was made by subjecting them to UV curing, it had a disadvantage in that the curing was difficult to proceed or required a long time.
Under these circumstances, it was pointed out that the resists using poly(hydroxystyrene)s (abbreviated as "PHS" hereinafter) were hopeful because of their high glass transition temperatures. Hitherto, PHS have been regarded as very low in compatibilities with other materials. However, it was recently found that PHS were compatible with compounds of specific structures when they were mixed in the specified range of proportions. Thus, the three-component resists of the type which contain PHS as a main component have been developed. Of the ortho-, the meta- and the para-isomers of PHS, only poly(p-hydroxystyrene) (p-PHS) is on the market, and so there is no room to chose an isomer other than p-PHS as a material of the resist composition from the viewpoints of availability and price. For instance, a three-component positive resist of the chemical amplification type which comprises p-PHS, an onium salt and a dissolution inhibitor is disclosed in Japanese Tokkai Hei 3-344686 (the term "Tokkai" as used herein means an "unexamined published patent application"). However, the resolution of this composition is 0.3 .mu.m at the highest and the sensitivity is not described explicitly. If it is analogized from the results of Examples 32 to 56 wherein other resins are used, the sensitivity will be 5 .mu.C/cm.sup.2 at the highest. Moreover, it has been revealed by our examination that the p-PHS using three-component resist compositions were inadequate for serving the practical use unless very severe restrictions were imposed thereon. This was because various unfavorable phenomena occurred depending on the proportions of individual ingredients to the composition as a whole and the kind of solvent used in the composition. For instance, some of the compositions had marked striation when they were applied in a film, some of them gave rise to the formation of quite a number of particles having a size on the order of a micron at the resist surface after the development, some on them caused the generation of craters having a diameter of the order of a submicron at the resist surface after the development, and some of them formed patterns of an inverted-taper shape. Accordingly, it is hard to say that the above-cited invention is adequate to the requirements of practical use.
As described above, there has not yet been known such a perfect chemical-amplification type positive resist for electron-beam or X-ray lithography as to satisfy all practical-use requirements. More specifically, the requirements are to have the sensitivity on the order of 1 .mu.C/cm.sup.2 to electron beams or on the order of 100 mJ/cm.sup.2 to X rays, to have the resolution on the order of 0.1-0.2 .mu.m, to have the heat resistance on such a level as to withstand the substrate processing with plasma, to control the weight loss due to decomposition of protective groups to less than 4%, to be applicable in a film of uniform thickness, to form patterns of an ideal shape, to ensure desirable residual film characteristics to the resist after the development, and so on.